The present invention relates to a signal level converting circuit and, more particularly, to such a circuit receiving an analog color signal to convert the signal level thereof and supplying the converted signal to a liquid crystal display device (called hereinafter an "LCD").
An LCD is advantageous in compact size, low voltage driving ability and low power consumption, as compared to a cathode ray tube (CRT) and a demand for it is expanding more and more. Considering the replacement of the CRT by the LCD device, since a color CRT is widely used and a graphic display program for personal computer is generally made for a color display, it is required to incorporate a color interface, i.e. an RGB (red, green, blue) interface, circuit into the LCD. Moreover, an analog RGB input interface circuit is preferable to receive analog color signal. The analog RGB signal has a signal level in which the reference level is 0 V and a signal amplitude is 0 to 0.7 V, and is thus required to be level-shifted to have a new reference level above 0 V in order to be subjected to a signal processing operation.
For this purpose, a conventional analog RGB interface circuit for the LCD has a capacitor to cut a DC component thereof and pass an AC component thereof and a resistor-divided circuit to level-shift the AC component passed through the capacitor. In this circuit, however, the DC component of the RGB signal is cut or eliminated, so that the black level thereof is changed with result that the brightness and darkness of an image are not constant. A pedestal clamping circuit is further required, accordingly. In the LCD, however, the driving voltages and/or timings are different form the those of the analog RGB signal. For this reason, the pedestal clamping circuit cannot be employed.
Therefore, such a level converting circuit as shown in FIG. 1 has been proposed. This circuit 50 includes an operational amplifier 51 and three resistors R51-R53 which are connected as shown. The analog (RGB) signal S-RGB is supplied through the resistor R52 to the inverting input terminal (-) of the amplifier 51 which has a negative feedback loop by the resistor R53. The non-inverting input terminal (+) of the amplifier 51 is supplied with a constant voltage VC. Accordingly, the signal S-RGB is inversely amplified by the amplifier 51 with reference to the voltage VC. Thus, the signal S-RGB is level-shifted by the voltage VC. The output signal Vo thus level-shifted is supplied to a signal processing circuit 4.
Recently, the LCD has been required to be used for a display device for a work station. In the work station, the analog RGB signal has a high frequency above 100 MHz for a purpose of high definition. For this reason, if the level converting circuit 50 shown in FIG. 1 is employed in the LCD for the work station, the phase of a feedback signal is changed remarkably due to the stray capacitance of the feedback resistor R53, so that the phase margin is not ensured to cause an oscillation. It is therefore considered to provide a capacitor for phase compensation, but in this case, the response of the operational amplifier 51 is too delayed to process the high frequency signal.